Polyethylene terephthalate decontamination

ABSTRACT

A process for removing contaminants from material containing polyethylene terephthalate (&#34;PET&#34;) which has been embrittled by mixing material containing PET with ethanediol having a temperature at or about the boiling point of ethanediol for a predetermined period of time, the process including the steps of: 
     (a) transesterifying the PET by reacting ethanediol having a temperature at or about the boiling point of ethanediol with the PET for a predetermined period of time to form a solution containing soluble short chain PET polymers and/or his (hydroxy ethyl) terephthalate ester (&#34;BHT&#34;); 
     (b) recovering short chain PET polymers and/or BHT and ethanediol; and 
     (c) hydrolysing the recovered short chain PET polymers and/or BHT at elevated pressure and temperature for a predetermined period of time to form an ethanediol solution and crystals of terephthalic acid.

This application is a continuation-in-part of U.S. patent application Ser. No 08/331,669 filed Nov. 16, 1994, now U.S. Pat. No. 5,504,121.

FIELD OF THE INVENTION

The invention relates a method for the decontamination of polyethylene terephthalate ("PET").

BACKGROUND OF THE INVENTION

PET is a thermoplastic polyester that can be formed from 1,2-dihydroxyethane ("ethanediol") and terephthalic acid by direct esterification to form bis (hydroxy ethyl) terephthatate ester ("BHT") which is then polymerised by catalysed ester exchange to useful polymers.

Traditionally, PET has been used extensively because it can be offered as an oriented film or fibre, has high tenacity, good electrical resistance and low moisture absorption together with a melting point of approximately 265 degrees Celsius.

For these reasons, its uses have been very diverse extending from blended with cotton for wash and wear fabrics, blended with wool for worsteds and suitings, packaging films and recording tapes and containers including soft drink containers.

There are a number of applications of PET where remelting and reforming is not permissible or gives inferior properties. The reuse of PET for these applications is best achieved by degrading the polymer into the original monomers namely ethanediol and terephthalic acid then reacting the monomers together to regenerate the original PET.

The known art (British Patent No. 610135) is to hydrolyse PET with either strong alkalis or acids. The alkalis must be neutralised with acid to produce terephthalic acid resulting in a significant cost of reagents and possible contamination of the product with alkali metal ions. The strong acids must be recovered and the ethanediol separated. Further the acids dissolve paper and pigments to give by-products which are difficult to separate.

It is known that PET will transesterify with ethanediol (British Patent No. 610136) but the product BHT, is not a convenient intermediate for subsequent purification to remove esters of contaminant acids such as benzoic and adipic acid.

Accordingly, the present invention is directed to an improved processes, for decontaminating PET.

DESCRIPTION OF THE INVENTION

It has been surprisingly found that ethanediol at temperatures at or about its boiling point reacts with the PET but does not react significantly with products often used in PET containing materials including paper and other plastics; for example, poly vinyl chloride ("PVC"), colouring dyes, pigments, mineral sands or clays ("contaminants").

Transesterification of PET with ethanediol can result in the PET becoming embrittled or solubilised depending on the period of time over which the transesterification is allowed to take place.

The present invention concerns processes of removing decontaminants from materials containing PFT by embrittling PET and subsequent treatment of the embrittled PET to remove further contaminants.

Accordingly, in a first form of this invention, a process is provided for removing contaminants from material containing PET including the following steps:

(a) transesterifying the material containing PET by mixing ethanediol having a temperature at or about the boiling point of ethanediol with the material for a predetermined period of time to form a mixture containing embrittled PET; and

(b) crushing the mixture and separating uncrushed material from the crushed material which contains PET.

According to a second form of the invention, a process is provided for removing contaminants from material containing PET which has been embrittled by mixing material containing PET with ethanediol having a temperature at or about the boiling point of ethanediol for a predetermined period of time, the process including the steps of:

(a) transesterifying the PET by reacting ethanediol having a temperature at or about the boiling point of ethanediol with the PET for a predetermined period of time to form a solution containing soluble short chain PET polymers and/or bis (hydroxy ethyl) terephthalate ester ("BHT");

(b) recovering short chain PET polymers and/or BHT and ethanediol;

(c) hydrolysing the recovered short chain PET polymers and/or BHT at elevated pressure and temperature for a predetermined period of time to form an ethanediol solution and crystals of terephthalic acid.

According to a third form of the invention, a process is provided for removing contaminants from material containing PET which has been embrittled by mixing material containing PET with ethanediol having a temperature at or about the boiling point of ethanediol for a predetermined period of time, the process including the steps of:

(a) transesterifying the PET by reacting ethanediol having a temperature at or about the boiling point of ethanediol with the PET for a predetermined period of time to form a solution containing soluble short chain PET polymers and/or his (hydroxy ethyl) terephthalate ester ("BHT"); and

(b) recovering short chain PET polymers and/or BHT and ethanediol.

According to a fourth form of the invention, a process is provided for removing contaminants from material containing PET which has been embrittled by mixing material containing PET with ethanediol having a temperature at or about the boiling point of ethanediol for a predetermined period of time, the process including the steps of:

(a) hydrolysing the PET in the mixture at elevated pressure and temperature for a predetermined period of time to form an ethanediol solution and crystals of terephthalic acid;

(b) esterifying the ethanediol solution and crystals of terephthalic acid to short chain PET polymers and/or bis (hydroxy ethyl) terephthalate ester ("BHT"); and

(c) recovering short chain PET polymers and/or BHT from the mixture,

Typically the step or steps involving transesterification with ethanediol are carried out at a temperature of within 10 degrees Celsius of the boiling point of ethanediol and more preferably at a temperature of within 5 degrees Celsius of the boiling point of ethanediol. Such temperature fluctuations do not substantially affect the lack of reactivity of the ethanediol with the contaminants.

Preferably, embrittlement is caused to occur after a period of between 20 minutes and 60 minutes and more preferably between 30 minutes and 50 minutes.

Embrittlement which takes place prior to the collapse of the original PET structure allows the PET to be crushed to separate it from contaminants. This allows separation to take place on the basis of size and density. Crushing to less than 1 millimeter is preferred. Crushing can be achieved by rollers or hammer mills or any other known technique for reducing the size of particles. Once crushed, the PET is separated from typical contaminants such as hydrocarbons and pigments from paper. Other substances giving rise to coloured products can be separated by screening, washing or density separation from other contaminants which have not become embrittled or have solubilised in the ethanediol such as paper fragments in ethanediol or pulp in water medium.

As indicated the PET rich fraction may be subsequently solubilised. In one preferred form of the invention this can take place preferably by filtration methods and more preferably by high pressure filtration methods. The addition of activated carbon or a combination of activated carbon and activated clay is used as an adsorbent for a wide variety of molecules such as dyes, pesticides, coloured polymers, etc.

Other foreign plastics with an alkane chain do not depolymerise in the boiling ethanediol but only melt so that they may be removed by either flotation on the denser ethanediol (1114 kilograms/m³), screened or filtered from embrittled PET or the short chain PET polymer and/or BHT solution. Proteins and polyamides do not react with ethanediol and are recovered with the other plastics. Other polyesters are also transesterified and contaminate the short chain PET polymer solution. It can be understood that this present invention does not claim that all the PET is converted to BHT only that the polymer chain length is short so that the terephthalate containing molecules are soluble in hot ethanediol to permit separation of foreign materials and rapid subsequent hydrolysis.

Following transesterification, the short chain PET polymers and/or BHT can be purified by crystallisation in water liberating excess ethanediol. The precipitate is filtered off. The filtrate containing ethanediol is recovered by fractional distillation from the additional water (British Patent No 610136). Water soluble impurities such as sugar and citric acid from soft drinks are partially removed in the filtrate and rejected as an involatile residue by distilling off the ethanediol.

Although it will be understood that the proportions of ethanediol to PET is not critical to the transesterification of the PET, in preferred forms of the invention the proportion of ethanediol to PET is at least 1:1. Excess ethanediol can be recovered by distillation techniques well known to those skilled in the art.

The short chain PET polymers and/or BHT are hydrolysed with water at elevated pressures and temperatures to give ethanediol and crystalline terephthalic acid. Typically, hydrolysis is achieved by dispersing the short chain PET polymers and/or BHT in a water slurry in a high pressure vessel and heating it to more than 180 degrees Celsius for 2 hours and allowing it to cool. This treatment gives good hydrolysis without the requirement for catalysts (which contaminate the product) and also gives good crystals of terephthalic acid with a high purity due to the greatly increased solubility at the higher temperature. The crystals are washed with water to remove ethanediol which is recovered as described before.

A number of water soluble substances (for example, citric acid, phosphoric acid, sodium chloride and sulphuric acid) in the feed stock will be present in the mixture of terephthalic acid and ethanediol and are separated by the filtration. Additionally, a wide variety of compounds such as protein, paper, fats and some pesticides will be hydrolysed and made water soluble relative to the very insoluble terephthalic acid.

Contamination of the short chain PET polymers and/or BHT by benzoic, adipic and sebacic acid is rectified because the free acids are approximately 1000 times more soluble in the wash water allowing some foreign polyester in the feed.

The hydrolysis can be improved by using a reaction of annular format where heating is followed by partial cooling to allow the solubility of terephthalic acid to cycle and assist crystallisation and removal from the aqueous phase. Further improvement in hydrolysis is given by allowing some boiling of the hydrolysing mixture then condensing and removing an ethanediol rich phase. The contaminant water soluble substances can be removed either prior to the transesterification by water washing and drying or after hydrolysis in the water and ethanediol solution and then are rejected in the residue after distillation of the water and ethanediol.

The ethanediol is readily purified by distillation and may be partially recycled to give both product for further polymerisation and enough product to react with fresh PET. Contaminant alcohols such as methanol from ethylene methyl acrylate or hexanediol from polyester are separated in the distillation step

DESCRIPTION OF THE DIAGRAM

One form of the invention is illustrated in the flow chart of FIG. 1.

Used PET is initially subjected to transesterification with boiling ethanediol for sufficient time to embrittle PET. Thereafter the mixture is passed through rollers which crush embrittled PET.

The non-crushed (ductile) components of the mixture are separated by screening from the remainder of the mixture to provide recovery of PET with partial decontamination.

If further decontamination is desired the fines can then subjected to a further transesterification process.

Activated carbon and clay is added during transesterification to adsorb contaminants. During this further transesterification process the PET. Is converted into short chain PET polymers and/or BHT which are soluble.

The resultant solution is subjected to a further separation by screening and/or high pressure filtration to remove contaminants such as activated carbon, clay, dye, PVC and glue.

The filtrate is hydrolysed with very pure water under elevated pressures and temperatures to form ethanediol and crystalline terephthalic acid. Acids (for example, benzoic and citric acids) remain in solution and are separated from crystalline terephthalic acid.

The ethanediol and terephthalic acid are esterified to short chain PET polymers and/or BHT. Activated carbon and activated clay are added and then any remaining contaminants for example PVC, glue and dye are removed by filtration.

The invention is illustrated by the following non-limiting examples.

EXAMPLE 1

1 kilogram of ethanediol was heated to 190 degrees Celsius then 600 grams of granulated PET soft drink bottle was added and heated for 45 minutes.

The embrittled PET was screened off then crushed between rolls to particles less than 1 millimeter, while paper label passed through the rolls unaffected.

EXAMPLE 2

The crushed PET of example 1 was divided into equal parts which were treated separately. One part was screened using ethanediol as an aid then the fines passing the screen were heated for one hour to convert the PET to soluble products. A mixture of 5 grams of activated carbon and 5 grams of activated clay were added and the mixture heated and stirred for 15 minutes then filtered and the product hydrolysed to give short chain PET polymers and/or BHT and ethanediol. The second part was screened with water then the product PET purified by washing which hindered settling to remove fine paper fibres and small particles.

EXAMPLE 3

A reactor was constructed from 3 meters of 21 millimeter pipe and a 1 liter bowl as an annulus with the plane of the ring held vertical. 1.5 liters of water was added to the reactor and the temperature brought to 200 degrees Celsius then a molten mixture of 100 grams of short chain PET polymers and/or BHT and 100 grams of ethanediol added to the water and maintained at 200 degrees Celsius for two hours by heating one segment of the annulus and allowing the rest to be partially cooled by the atmosphere. The apparatus was then cooled and crystalline terephthalic acid and ethanediol recovered from the water suspension.

EXAMPLE 4

A sample of terephthalic acid (100 grams) was taken and dispersed in 1 liter of ethanediol and heated at the boiling point with reflux to remove water of reaction for 4 hours. A mixture of 2 grams of activated carbon and 2 grams of activated clay was added and stirred for 30 minutes then filtered off to remove PVC, PET dyes etc, and then the excess ethanediol was distilled off and the polymerisation was completed by heating to 270 degrees Celsius under vacuum to produce a PET polymer.

EXAMPLE 5

1 kilogram of polyester fibre (PET) containing cotton and polyamide fibre was immersed in 2 kilograms of ethanediol at 190 degrees Celsius and the temperature maintained in the range of 190 to 196 degrees Celsius for 45 minutes. The fibre mass which now contained short chain PET polymers was separated from the liquid by screening. The short chain PET polymers were ground to powder, leaving the cotton largely undamaged. The polyamide dissolved in the ethanediol. The powder was suitable for repolymerisation to polyester fibre.

The process described above can be used to effectively treat a variety of materials which contain PET to give pure monomers for repolymerisation. This discovery allows the use of dirty whole bottles with unsorted foreign plastics and attached labels and tops with some foreign bottles and plastic debris. The process can also be used to recover PET monomers from used X-ray film or mixed fabrics.

Since modifications to the steps described are various and obvious to those skilled in this art it is to be understood that this invention is not limited to the particular embodiments described. 

The claims defining the invention are as follows:
 1. A process for removing contaminants from material containing polyethylene terephthalate ("PET") including the following steps:(a) transesterifying the material containing PET by mixing ethanediol having a temperature at or about the boiling point of ethanediol with the material for a predetermined period of time to form a mixture containing embrittled PET; and (b) crushing the mixture and separating uncrushed material from the crushed material which contains PET.
 2. The process of claim 1, wherein step (a) is carried out at a temperature of within 10 degrees Celsius of the boiling point of ethanediol.
 3. The process of claim 1, wherein step (a) is carried out at a temperature of within 5 degrees Celsius of the boiling point of ethanediol.
 4. The process of claim 1, wherein the proportion of ethanediol to PET is at least 1:1.
 5. The process of claim 1, wherein the predetermined period of time in step (a) is between 20 minutes and 60 minutes.
 6. The process of claim 1, wherein the predetermined period of time in step (a) is between 30 minutes and 50 minutes.
 7. The process of claim 1, wherein step (b) the PET is crushed to a size of less than 1 millimeter.
 8. The process of claim 1, wherein step (b) includes the step of density separation.
 9. A process for removing contaminants from material containing polyethylene terephthalate ("PET") which has been embrittled by mixing material containing PET with ethanediol having a temperature at or about the boiling point of ethanediol for a predetermined period of time, the process including the steps of:(a) transesterifying the PET by reacting ethanediol having a temperature at or about the boiling point of ethanediol with the PET for a predetermined period of time to form a solution containing soluble short chain PET polymers and/or bis (hydroxy ethyl) terephthalate ester ("BHT"); (b) recovering short chain PET polymers and/or BHT and ethanediol; and (c) hydrolysing the recovered short chain PET polymers and/or BHT at elevated pressure and temperature for a predetermined period of time to form an ethanediol solution and crystals of terephthalic acid.
 10. The process of claim 9, wherein step (a) is carried out at a temperature of within 10 degrees Celsius of the boiling point of ethanediol.
 11. The process of claim 9, wherein step (a) is carried out at a temperature of within 5 degrees Celsius of the boiling point of ethanediol.
 12. The process of claim 9, wherein the proportion of ethanediol to PET in step (a) is at least 1:1.
 13. The process of claim 9, wherein the predetermined period of time in step (a) is at least one hour.
 14. The process of claim 9, wherein the predetermined period of time in step (a) is at least two hours.
 15. The process of claim 9, wherein the predetermined period of time in step (a) is about two hours.
 16. The process of claim 9, wherein the PET and ethanediol are recovered in step (b) by filtration.
 17. The process of claim 16, wherein the filtration is high pressure filtration.
 18. The process of claim 9, wherein the filtration includes the addition of activated carbon or a combination of activated carbon and activated clay.
 19. A process for removing contaminants from material PET which has been embrittled by mixing material containing PET with ethanediol having a temperature at or about the boiling point of ethanediol for a predetermined period of time, the process including the steps of:(a) transesterifying the PET by reacting ethanediol having a temperature at or about the boiling point of ethanediol with the PET for a predetermined period of time to form a solution containing soluble short chain PET polymers and/or bis (hydroxy ethyl) terephthalate ester ("BHT"); and (b) recovering short chain PET polymers and/or BHT and ethanediol.
 20. The process of claim 19, wherein step (a) is carried out at a temperature of within 10 degrees Celsius of the boiling point of ethanediol.
 21. The process of claim 19, wherein step (a) is carried out at a temperature of within 5 degrees Celsius of the boiling point of ethanediol.
 22. The process of claim 19, wherein the proportion of ethanediol to PET in step (a) is at least 1:1.
 23. The process of claim 19, wherein the predetermined period of time in step (a) is at least one hour.
 24. The process of claim 19, wherein the predetermined period of time in step (a) is at least two hours.
 25. The process of claim 19, wherein the predetermined period of time in step (a) is about two hours.
 26. The process of claim 19, wherein the PET and ethanediol are recovered in step (b) by filtration.
 27. The process of claim 26, wherein the filtration is high pressure filtration.
 28. The process of claim 26, wherein the filtration includes the addition of activated carbon or a combination of activated carbon and activated clay.
 29. A process for removing contaminants from material containing PET which has been embrittled by mixing material containing PET with ethanediol having a temperature at or about the boiling point of ethanediol for a predetermined period of time, the process including the steps of:(a) hydrolysing the PET in the mixture at elevated pressure and temperature for a predetermined period of time to form an ethanediol solution and crystals of terephthalic acid; (b) esterifying the ethanediol solution and crystals of terephthalic acid to short chain PET polymers and/or bis (hydroxy ethyl) terephthalate ester ("BHT"); and (c) recovering short chain PET polymers and/or BHT from the mixture.
 30. The process of claim 29, wherein the PET and ethanediol are recovered in step (c) by filtration.
 31. The process of claim 30, wherein the filtration is high pressure filtration.
 32. The process of claim 30, wherein the filtration includes the addition of activated carbon or a combination of activated carbon and activated clay. 